1. Field of Invention
The invention relates to an exhaust gas control apparatus for an internal combustion engine provided, for example, with a three-way catalyst in an exhaust passage. The invention also relates to an exhaust gas purification method.
2. Description of Related Art
A three-way catalyst (also referred to simply as “catalyst” in this specification) used to purify exhaust gas from an internal combustion engine has been conventionally arranged in an exhaust passage of the internal combustion engine. This three-way catalyst has an O2 storage function in which it stores oxygen (hereinafter, this function will be referred to as an “oxygen storing function” and the amount of oxygen stored in the catalyst will be referred to as the “amount of stored oxygen”). The three-way catalyst oxidizes unburned components such as HC and CO using oxygen that is stored by the catalyst when the air-fuel ratio of gas that flows into the catalyst is rich, and reduces oxides of nitrogen (NOx) by storing oxygen from the NOx when the air-fuel ratio of the gas that flows into the catalyst is lean. As a result, the three-way catalyst is able to purify harmful components such as unburned components and oxides of nitrogen even when the air-fuel ratio of the internal combustion engine is not at the stoichiometric air-fuel ratio.
Because the operation state of the internal combustion engine is continually changing, the air-fuel ratio of the internal combustion engine continually changes between rich and lean. Meanwhile, the amount of oxygen stored in the catalyst changes between “0” and the maximum storable amount of oxygen. Accordingly, if the air-fuel ratio of the internal combustion engine is rich when the amount of oxygen stored in the catalyst is near “0”, then unburned components flow out from the catalyst without being sufficiently oxidized (i.e., there is not enough oxygen stored in the catalyst to sufficiently oxidize them). Conversely, if the air-fuel ratio of the internal combustion engine is lean when the amount of oxygen stored in the catalyst is near the maximum storable amount of oxygen, then oxides of nitrogen flow out from the catalyst without being sufficiently reduced (i.e., there is not enough room in the catalyst to store the oxygen from the NOx). Therefore, it is preferable to control the air-fuel ratio of the internal combustion engine so that the amount of oxygen stored in the catalyst for effectively purifying unburned components and oxides of nitrogen is a predetermined amount.
Meanwhile, in order to both purify exhaust gas immediately after starting the internal combustion engine and further improve exhaust gas control performance after the internal combustion engine has completely warmed up, a construction has recently been employed in which a first catalyst having a relatively small capacity, also referred to as a “start converter”, is disposed in the exhaust passage of the internal combustion engine while a second catalyst having a relatively large capacity, also referred to as an “under floor converter”, is disposed in the exhaust passage downstream of the first catalyst. In this case, because the first catalyst is disposed closer to the exhaust port of the internal combustion engine than the second catalyst so that hot exhaust gas flows into that first catalyst, it is warmed up quickly after starting the engine and therefore displays good characteristics in purifying the exhaust gas early after startup. On the other hand, the second catalyst requires more time than the first catalyst to warm up, but once it has warmed up, it displays excellent characteristics in purifying the exhaust gas because of its large capacity.
In this way, because unburned components and oxides of nitrogen are able to be effectively purified when the first catalyst and the second catalyst are disposed in series in the exhaust passage of the internal combustion engine, an exhaust gas control apparatus disclosed in Japanese Patent Laid-Open Publication No. 2001-234787 calculates the amount of oxygen stored in the first catalyst and controls the air-fuel ratio of the internal combustion engine so that the calculated amount of oxygen stored in the first catalyst matches a target amount of stored oxygen (e.g., an amount that is approximately half of the maximum storable amount of oxygen of the first catalyst). In addition, the exhaust gas control apparatus reduces the target amount of oxygen stored in the first catalyst when an output from an air-fuel ratio sensor disposed downstream of the second catalyst indicates that the exhaust gas flowing out from the second catalyst is lean, and increases the target amount of oxygen stored in the first catalyst when the output from the air-fuel ratio sensor disposed downstream of the second catalyst indicates that the exhaust gas flowing out from the second catalyst is rich.
Here, when the exhaust gas flowing out from the first catalyst is lean, the amount of oxygen stored in the second catalyst is near the maximum storable amount of oxygen, so there is not much room in the second catalyst to store oxygen. At this time, according to the foregoing apparatus, the target amount of oxygen stored in the first catalyst is reduced and the air-fuel ratio of the internal combustion engine is controlled to be rich. Accordingly, the room in the first catalyst to store oxygen increases. Therefore, the catalyst apparatus, when regarded as including both the first catalyst and the second catalyst, is able to ensure enough room to store oxygen.
On the other hand, when the exhaust gas flowing out from the second catalyst is rich, the amount of oxygen stored in the second catalyst is near “0”, so there is not much oxygen in the second catalyst to release. At this time, according to the foregoing apparatus, the target amount of oxygen stored in the first catalyst is increased and the air-fuel ratio of the internal combustion engine is controlled to be lean. Accordingly, the amount of oxygen in the first catalyst that can be released increases. Therefore, the catalyst apparatus is able to ensure that there is enough oxygen in the catalyst so that some can be released.
Accordingly, in the foregoing apparatus, even if the air-fuel ratio of the exhaust gas flowing into the first catalyst (i.e., the air-fuel ratio of the internal combustion engine) temporarily becomes quite lean or rich due to engine acceleration or deceleration, the catalyst apparatus, when regarded as including both the first catalyst and the second catalyst, is able to stably maintain both enough room for oxygen to be stored and enough oxygen so that some can be released, and therefore effectively purify harmful components such as unburned components and oxides of nitrogen.
However, the air-fuel ratio sensor of the foregoing apparatus is disposed in the exhaust passage a predetermined distance downstream of the farthest downstream point of the second catalyst, such that it takes a certain amount of time, depending on the flow rate of the exhaust gas, for the exhaust gas that flows out from the farthest downstream point of the second catalyst to reach the air-fuel ratio sensor. Therefore, when the air-fuel ratio (i.e., characteristics) of the exhaust gas flowing out of the farthest downstream point of the second catalyst changes, there is a certain amount of delay until the exhaust gas having the changed air-fuel ratio reaches the air-fuel ratio sensor. There is also a certain amount of delay between the time the exhaust gas having the changed air-fuel ratio reaches the air-fuel ratio sensor and the time that the air-fuel ratio sensor changes its output to reflect that change in the air-fuel ratio.
In view of these facts, with the apparatus of the aforementioned publication, in particular, when the air-fuel ratio of the internal combustion engine changes abruptly such that the air-fuel ratio (i.e., characteristics) of the exhaust gas flowing out from the second catalyst changes abruptly, there is a delay in the change of the target amount of oxygen stored in the first catalyst because of the inevitable delay in the change of the output from the air-fuel ratio sensor. As a result, the catalyst apparatus may not be able to sufficiently maintain both enough room for oxygen to be stored and enough oxygen so that some can be released, and therefore may not be able to reliably purify harmful components such as unburned components and oxides of nitrogen.